1. Field of the Invention
The present invention relates to a dispersion compensation device, which is suitably used for compensating chromatic dispersion in an optical communication system.
2. Description of the Related Art
Our society has reached a stage, so called, an information society. Optical fiber communication system, which is capable of transmitting a large volume and a variety of information at a high speed, is evaluated as an essential system indispensable for our lives. Generally, the optical fiber communication system includes a signal source for generating optical signals, an optical fiber transmission line for transmitting the optical signals and an optical receiver for detecting and demodulating the optical signals.
In this optical fiber communication system, when optical signals within a specific wavelength range are transmitted through an optical fiber having positive chromatic dispersion, a delay is generated because the propagation velocity of optical signals with longer wavelength components is lower than that of optical signals with shorter wavelength components. The waveform of the optical signals deteriorates due to the delay. Particularly, in the case where a number of channels are transmitted in a wide wavelength range, a chromatic dispersion compensation that precisely compensates such differences in propagation velocity (chromatic dispersion) is required. In particular, in a DWDM (Dense Wavelength Division Multiplexing) communication system, compensation of chromatic dispersion as described above; i.e., dispersion slope compensation is indispensable.
Conventionally, a dispersion compensation fiber (DCF) is used for the purpose of the dispersion compensation. However, the DCF is expensive and compensates only a fixed compensation amount.
Also, the following small dispersion compensation device has been proposed as shown in FIG. 8 (for example, refer to patent document 1). That is, the light propagating through a single mode fiber (SMF) is converted into the light of higher mode, and allowed to propagate through a multi-mode fiber (MMF), thereby dispersion compensation is carried out.
In a dispersion compensation device 100 shown in FIG. 8, it is arranged so that a beam of 0th mode light, which propagates through a single mode fiber 101, is allowed to pass through a cylindrical member 102 of which inside surface is formed of a total reflection member 102a to generate interference with each other; and thereby, the 0th mode light is converted into a multi-mode light, and the converted multi-mode light is allowed to propagate through a multi-mode fiber 104 via a lens 103. That is, it is arranged so that, by utilizing such a characteristic of the multi-mode fiber 104 itself that the dispersion slope characteristic varies depending on the status of mode of the multi-mode light, the chromatic dispersion of the light propagating through the single mode fiber 101 is compensated.
[Patent document 1] Published Japanese translation of PCT international application, No. 2002-507778
However, the conventional DCF as described above has such a configuration that the dispersion compensation amount is controlled based on the length of the optical fiber. Accordingly, the dispersion characteristics, which are compensated by one DCF, are fixedly determined. Therefore, in the case where the dispersion amount changes as time passes, dispersion compensation following the changes is hardly carried out. Therefore, dispersion compensation corresponding to a high-speed transmission is hardly achieved. Also, when it is intended to carry out the dispersion compensation of an optical transmission path by means of the DCF, dispersion compensation fibers of a length, which corresponds to each of the optical fibers employed as the optical transmission paths respectively, have to be formed corresponding to each of the optical transmission paths. Thus, there also resides a problem to be improved in an aspect of cost.
In the above-described dispersion compensation device 100 shown in FIG. 8 also, same as the case of the DCF, since the status of mode of the multi-mode light to be converted depends on the length of the cylindrical member 102, the dispersion compensation devices 100 having a compensation amount respectively corresponding to each of the optical fibers employed as the optical transmission paths have to be designed. Therefore, dispersion compensation corresponding to a high-speed transmission is hardly carried out. There also resides a problem to be improved in an aspect of cost.
Therefore, to cope with the above-described situation, it is preferred to employ a variable dispersion compensation device capable of changing the compensation amount corresponding to the length of the optical transmission path and/or transmission speed.